#include <stdio.h>

int main() {
    int input = 0;
    scanf("%d", &input);
    if(input>100||input<0)
    input=120;
    int level = input / 10;
    switch (level) {
        case 0:
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
            printf("E");
            break;
        case 6:
            printf("D");
            break;
        case 7:
            printf("C");
            break;
        case 8:
            printf("B");
            break;
        case 9:
        case 10:
            printf("A");
            break;
        default:
            printf("F");
    }
    return 0;
}
